To examine the role of long range afferents and the possibility that ineffective synapses provide standby inputs which may be turned on physiologically or by growth processes following injuries which destroy the normal input to a region. To exploit our breakthrough into the substantia gelatinosa where we can activate and record cells in this structure. This we shall do in the following ways: a) To investigate by anatomical and physiological means the distribution of afferents into the region. In particular, we suspect that there is a segregation of different types of afferent fibre. b) To examine the effects of substantia stimulation on the transmission of impulses from dorsal roots to motor neurones. c) To examine the effect of substantia cell discharge on the firing patterns of single cells in dorsal horn which transmit to the brain and to the ventral horn. d) To examine the transmitting abilities of the fine terminal branches of arriving afferent fibres while varying the activity of substantia cells. e) To study the pharmacology of such cell discharge. BIBLIOGRAPHIC REFERENCES: BASBAUM, A.I. & WALL, P.D. (1976) Chronic changes in the response of cells in adult cat dorsal horn following partial deafferentation: the appearance of responding cells in a previously non-responsive region. Brain Res., 116: 181-204. WALL, P.D. (1977) The presence of ineffective synapses and the circumstances which unmask them. Phil. Trans. Roy. Soc. B., in press.